<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <!-- 外联样式 -->
    <link rel="stylesheet" href="theme.css">
    <!-- 内嵌样式 -->
    <style>
    .text p{
        color: red;
    }
    div p {
        color: blue;
    }
    .container #main p{
        color: green;
    }
    .btn {
        background-color: black;
    }
    </style>
</head>
<body>
    <div class="text">
    <p>Hello</p>
  </div>
  
  <div class="container">
    <div id="main">
      <p>Hello</p>
    </div>
  </div>
<!-- 行内样式,少用 -->
  <button class="btn" style="background: pink;">Click</button>
</body>
</html>